Stents, such as braided or knitted stents for surgical implantation in body lumens (tubular vessels), are known for repairing or strengthening the vessels. A stent essentially is a hollow tube that supplements the body lumen. With respect to the medical condition of stenosis, in which a body lumen tends to collapse or otherwise close, the stent supports the wall of the vessel to prevent it from collapsing or closing. A blood vessel that is narrowed due to the build up of intra-vascular plaque is one example of a stenosis. With respect to the medical condition of aneurism, in which a body lumen is weakened and cannot properly withstand the internal pressure within the vessel and bulges out or ruptures, a stent graft serves essentially the opposite function in that it substitutes for or supplements a weakened portion of the vessel. Stents are known for insertion in blood vessels, bile ducts, colons, trachea, esophagi, urethra, ureters, nasal passages, ductal systems, etc.
Stents are known that are fabricated from rigid, but flexible materials that, when bent by force, tend to retain the bent shape. Such stents may be inserted into the body lumen in an unstressed radially minimal shape while mounted over a deflated balloon. When the stent is in situ, the balloon is inflated in order to radially expand the stent, which will then retain the radially expanded shape after the balloon is deflated and removed.
Another type of stent is termed a self-expanding stent. Self-expanding stents can be compressed radially, but will expand to their original shape once the radially constrictive force is removed. Some types of self-expanding stent are formed from materials that are superelastic or have shape memory characteristics. Such stents are commonly made of Nitinol, a biocompatible alloy that, depending on its chemical composition and thermomechanical history, may be either a shape memory material or a superelastic material. The ULTRAFLEX stent manufactured and sold by Boston Scientific Corporation is an example of a knitted Nitinol stent.
Another type of self-expanding stent that reverts to its original shape due to an elastic deformation when radially compressed is exemplified in U.S. Pat. No. 4,655,771, issued to Wallsten and incorporated herein by reference. Wallsten discloses a self-expanding, braided surgical dilator stent particularly adapted for coronary dilation, but which can be adapted for use in other body vessels. That patent discloses a stent generally in accordance with the stent 10 shown in FIG. 1A. It comprises a hollow tubular member, the wall of which is formed of a series of individual, flexible, thread elements 12 and 14, each of which extends helically around the central longitudinal axis of the stent. A first subset of the flexible thread elements 12 have the same direction of winding and are displaced relative to each other about the cylindrical surface of the stent. They cross a second plurality of helical thread elements 14 which are also displaced relative to each other about the cylindrical surface of the stent, but having the opposite direction of winding. Accordingly, as shown in FIG. 1A, the threads 12 of the first subset cross the threads 14 of the second subset at crossing points 16.
As the stent is axially stretched, i.e., as the longitudinal ends 18 and 20 are forced away from each other, the diameter reduces, as shown in FIG. 1B. Likewise, if the wall of the stent is radially constricted so as to reduce the stent's diameter, the stent elongates. In other words, radial constriction and axial elongation go hand in hand. When the force is released, the stent tends to spring back to its resting diameter and length.
Bioabsorbable stents also are known in the prior art, including bioabsorbable braided self expanding stents of the type generally disclosed in the aforementioned Wallsten patent. Bioabsorbable stents are manufactured from materials that dissolve over an extended period of time when exposed to bodily fluids and are absorbed into the surrounding cells of the body.
Various bioabsorbable materials that are suitable for fabricating stents are known in the prior art, including polymers such as poly-L,D-lactic acid, poly-L-lactic acid, poly-D-lactic acid, polyglycolic acid, polylactic acid, polycaprolactone, polydioxanone, poly(lactic acid-ethylene oxide) copolymers, or combinations thereof. Vainionp et al., Prog Polym. Sci., vol. 14, pp. 697–716 (1989); U.S. Pat. Nos. 4,700,704, 4,653,497, 4,649,921, 4,599,945, 4,532,928, 4,605,730, 4,441,496, and 4,435,590, all of which are incorporated herein by reference, disclose various compounds from which bioabsorbable stents can be fabricated.
Most, if not all, stents need to be radially constricted, i.e., reduced in diameter from their deployment radius, so that they can be inserted into the body lumen. Then, once they are in situ, the stent can be released and radially expanded.
Various insertion apparatus for delivering a stent into a body lumen in a radially constricted state and then releasing stent so that it self expands within the body lumen are available. In one popular design illustrated in FIG. 2 and exemplified, for instance, in U.S. Pat. No. 5,026,377, the delivery apparatus comprises an inner core tube 5 surrounded by a concentric outer tube 1. The outer tube is shorter than the inner tube so that the inner tube can extend from the outer tube at both ends of the delivery device. A handle 6 typically is provided at the proximal end of the inner tube 5. Another handle 2 is provided at the proximal end of the outer tube. The inner tube 5 is slidable within the outer tube by relative manipulation of the two handles. A stent 11 is loaded on the delivery apparatus trapped between the core and the outer tube near the distal end of the delivery apparatus.
The inner tube 5 may be hollow and adapted to accept a guide wire 8 which, as is well known in the related arts, can be used to help guide the distal end of the delivery device to the stent deployment site in the body lumen 4.
During stent delivery, a physician typically will make an incision in the body lumen at a location remote from the stent deployment site and then guide the stent delivery device into the body lumen until the distal end of the stent delivery device is at the stent deployment site. The outer tube is then pulled back out while the inner tube is held in position. Accordingly, the outer tube slides over the stent, thus releasing it from radial constriction, whereby the stent radially expands and contacts the wall of the body lumen and is held in place by frictional force between the lumen wall and the stent body resulting from the radial expansion force of the stent. The stent is now fully deployed and the delivery device can be retracted.
Sometimes, after the stent has been partially released from the delivery device, the physician may decide that the stent is not properly positioned. In such a case, the physician will push the outer tube distally (or draw the inner tube proximally) to retract the stent back into the outer tube and then move the delivery device to a better position to redeploy the stent. Generally, once a self expanding stent is fully released from the delivery device, it cannot be recaptured within the delivery device because the proximal end of the stent has been released from radial constriction and is now bigger than the outer tube and, there is no way to radially constrict it again.
It can be seen from the description above that, during release of the stent in the body lumen, the outer tube must move proximally while the stent and inner tube remain stationary. Likewise, in the case of the need to retract the stent back into the outer tube, the stent must remain stationary as the outer tube is pushed distally back over the stent. However, since the stent, and especially self expanding stents, may exert a radially outward force in the inner wall of the outer tube, the stent may get pulled along with the outer tube due the frictional engagement of the stent and the outer tube.
Aforementioned U.S. Pat. No. 5,026,377 discloses a technique adapted to ensure that the stent remains stationary and fixed to the inner tube when the outer tube is moved relative thereto. That patent discloses a gripping member on the inner tube adjacent the stent, the gripping member comprising a high friction, enlarged diameter portion of the inner tube. The material of the gripping member may take a set around the stent, i.e., locally deform around the threads of the stent as the stent is compressed against the gripping member by the outer tube thereby gripping the stent. The surface of the gripping member may or may not be roughened to increase its ability to grip the stent.